For safety reasons in electrically operated installations where water is present, it has been proposed to use pressure operated electrical switches which are actuated by a manually operable bellows unit connected to the pressure switch by a non-conductive small bore tube. The switch can then be located near the electrical circuitry which it controls, and the bellows unit can be located remotely from the switch.
Since pressure may be lost from the bellows unit, tube or switch if they remain pressurized for a long time, it has been proposed to employ a bistable mechanism in the switch, so that one pulse of pressure changes the state of the switch and a subsequent pulse reverts the switch to its original state. The previously proposed bistable mechanism involves use of a lever connected to a bellows of the pressure switch which is connected to a crank mounted by a ratchet on a shaft. Thus repeated operations of the bellows index round the shaft. The shaft has a detent mechanism to make its indexing more positive. One or more cams are mounted on the shaft and operate the push-buttons of one or more microswitches. Thus, as the shaft is indexed around, the states of the microswitches are changed.
Problems with this arrangement are that the mechanism is complicated, and despite the use of the detent mechanism, the shaft may not index around properly.